RETROSPECT 351 



natural rather than supernatural or metaphysical 

 explanation. In other words, Greek interpreta- 

 tions of the initial i)assage from the inorganic to 

 the organic world, of the origin of life, of com- 

 petition and the struggle for existence, of the 

 origin of adaptations, are from the beginning 

 naturalistic, although eventually they become 

 philosophical and constitute an important part 

 of the Greek philosophy of Nature. 



Nevertheless, the philosophical environment 

 of the evolution idea is seen gradually shaping 

 itself in a better understanding of the relations 

 of Design and of Causation, while the natural- 

 istic environment is seen expanding step by step 

 with the biological sciences. Two of Aristotle's 

 philosophical principles, lying midway between 

 physics and metaphysics, have exerted a pro- 

 found and very misleading influence even down 

 to the present day. I refer first to his 'perfecting 

 tendency,' which led Leibnitz and all his natural- 

 istic and speculative followers away from the 

 search for a natural cause of Adaptation; why 

 seek for a natural cause of Adaptation through 

 experiment and observation when we philosophi- 

 cally assume such a cause in an 'internal perfect- 

 ing tendency'? The second philosophical prin- 

 ciple of Aristotle, embodied in his idea of 'unity 

 of type,' also exerted a deterrent influence on 

 observation and, as finally developed in the mind 



